Increasing traffic density and new construction continue to narrow available traffic space, in particular in densely populated areas. Available parking space becomes more limited, and the search for a suitable parking space, together with the constant increase in traffic, places stress on the driver. Therefore, semi-autonomous parking assistants (SPA) have been developed to help the driver when parking. In this way, the decision as to whether or not the car will fit into a given parking space is made easier, or even taken over entirely by the system. Subsequently, the system gives the driver instructions for speedy parking. Though applicable to any parking spaces, the present invention and the problem on which it is based are explained with reference to the process of parking a motor vehicle in a parallel parking space.
German Published Patent Application No. 29 32 118 describes a system for parking assistance that measures the distances from obstacles in the environment of the motor vehicle, and warns the driver of collision with these obstacles. However, this system does not facilitate parking as such, but rather only warns the driver of collisions with other vehicles or objects.
German Published Patent Application No. 38 13 038 describes a parking system that measures the parking space while driving by it, and assists the driver during parking. The parking trajectory planned by the parking system is made up of straight segments and circular arcs, i.e., the driver steers only while stationary. The points situated in between, known as turning points, cannot be precisely stopped at by the driver. The driver will make mistakes both during transverse guiding (setting and holding of the desired steering wheel angle) and also during longitudinal guiding (observing the stopping or turning point). Moreover, errors due to the measurement of the parking space will additionally have the result that stopping points must be corrected. The higher the degree of robustness of the overall system against such errors, the higher its degree of acceptance by the driver will be.
On the one hand, the process of parking requires that the driver actuate the steering wheel in order to effect the steering, and on the other hand he has to move the vehicle forward or backward through acceleration. If necessary, the vehicle is also stopped at a point provided for this purpose in order to enable execution of a change of direction by turning the steering wheel or by changing gear. In general, the instructions are provided in such a manner that the driver is not supposed to drive the vehicle while he is turning the steering wheel, so that while steering is taking place the vehicle is at a standstill, and when the vehicle is in motion the steering angle is generally not to be further modified. In particular, unambiguous steering positions are achieved by instructing the driver to carry out a full steering turn while at a standstill. The driver is thus directed to follow a parking trajectory that is made up of straight segments, or, if necessary, circular arcs that are as tight as possible. When the driving and the steering are separated, various characteristics of the driver with respect to his reaction time can be excluded, because the individual actions are to be executed by the driver independent of operating speed. However, this procedure is fairly laborious for the driver. German Published Patent Application No. 10 2004 047 484 describes a method for outputting parking instructions to a driver of a vehicle, in which a steering instruction for turning the steering is outputted, and, after a specified distance after the outputting of the steering instruction, a corrective instruction is then outputted if, at an end of the specified distance, at least a specified steering angle of the steering system has not been set.
This method has the particular characteristic that at first a steering instruction for turning the steering is outputted to the driver, but it is not checked until after a specified distance has been traveled whether a specified steering angle has now been set. This gives the driver the opportunity to turn the steering while moving. In this way, steering while at a standstill is avoided. In addition, within a certain tolerance range the driver can also turn the steering wheel with the speed to which he is accustomed. He does not have to be limited to a specified steering speed. This makes steering easier, in particular while moving. If steering while moving is allowed, a lower torque has to be applied for the actuation of the steering system, because steering is easier while the vehicle is rolling than when it is at a standstill. As a whole, the parking process is more comfortable for the driver, because it corresponds more closely to the standard parking procedure without assistance, in which steering and moving generally take place together. In addition, the smoother flow of the procedure can reduce the time required for the parking process. In this way, the roadway, which may be blocked at least partly by the parking process, is cleared again more quickly for subsequent traffic. Thus, the specified distance defines a turning zone within which the driver can carry out a required steering modification. Because at the end of the turning zone it is nonetheless checked whether a turning has taken place, a correction can still be carried out for the case in which the driver has not sufficiently actuated the steering wheel. Even for the case in which the driver has not acted correctly, at this time in general a correction is still possible such that the parking process can be continued and brought to a successful conclusion. However, this method has the disadvantage that often after a specified distance has been traveled a correction will take place, and the driver is always guided back to an initially calculated optimal trajectory.
German Published Patent Application No. 103 31 235 discloses a driver assistance device, in particular for parking a motor vehicle, having an output unit for outputting driving instructions to a driver, in which the driving instructions indicate to the driver a driving range between two trajectories that are calculated such that the vehicle can be moved within the driving range in order to freely select a path into the parking space.
In this way, the driver need not follow a single prespecified ideal line, while nonetheless being assured that there is no risk of collision.
Example embodiments of the present invention for outputting parking instructions may provide that a high degree of robustness is ensured by taking possible errors into account in the trajectory planning. A system distinguished by a robust trajectory planning of this sort provides larger tolerance ranges for the driver's actions (maintenance of stopping path and steering wheel angle), and simultaneously reduces the number of unsuccessful, aborted parking attempts due to driver error or mistakes in measuring the parking space. In this manner, the acceptance of the overall system by drivers is increased.
In the method according to example embodiments of the present invention for outputting parking instructions to a driver of a motor vehicle, the parking process includes a multiplicity of sections, each having a constant specified steering angle, there being situated between each two sections a stopping point within a stopping path that is to be approached after a corresponding stopping instruction.
First a trajectory is calculated, and the resulting required steering angle is specified to the driver. On the basis of a trajectory planning using the current steering angle and the current position, at each point in time it is checked whether the parking process can be successfully carried out, i.e. whether the trajectory selected at that moment by the driver leads to a successful conclusion of the parking process. As long as this is the case, no instruction is issued for the interruption, correction, or abortion of the parking process. This provides that a maximum degree of tolerance of errors in steering can be achieved, not only when the steering angle is first set but also during travel. If during curved travel the driver makes a change to the steering, this error is accepted, i.e., further planning continues with the current value as long as successful parking is possible.
The particular robustness of the device or of the method is achieved in that a tolerance band is defined for the stopping path and/or for the steering angle, and for each section a corresponding stopping instruction is defined, corresponding to a specified stopping path and/or a corresponding specified steering angle, taking the tolerance band into account.
This robustness is noticeable in particular in relation to errors of the following type:                driver errors in maintaining the steering wheel angle during travel in a straight line or on a curved path;        driver errors when stopping at the end of a section;        errors in measuring the boundaries of the parking space.        
This robustness can be achieved in particular through the following measures:
The trajectory planning is carried out not only at the beginning of the parking process, but is executed cyclically whenever the vehicle is in motion.
The robustness with respect to steering wheel angle deviations is achieved in that the cyclical trajectory planning during vehicle movement always takes into account the current steering wheel angle for the current section, and a correction of the steering wheel angle is required only if a successful termination of the parking process is no longer possible with the current steering wheel angle. In this way, the maximum possible tolerance is achieved with respect to errors in maintaining the steering wheel angle.
Robustness with respect to maintaining the stopping points (longitudinal guiding) or also with respect to errors in parking space measurement is achieved in that what is followed is not an optimal trajectory, but rather a selected trajectory such that, even if the next stopping point, or optionally all following stopping points, are not observed, said trajectory will reliably result in a successful termination of the parking process as long as the intermediate point is situated within the possible range of intermediate points of the current specified trajectory.
According to example embodiments, during a driving instruction it is cyclically checked whether the current steering angle lies within a valid range of possible steering angles for a successful parking. A corrective instruction for stopping and for setting a corrected steering angle is outputted only if the relevant steering angle lies outside the range of valid steering angles.
According to example embodiments, immediately after the stopping of the vehicle at a respective stopping point it is checked whether the stopping path lies within a valid range of possible stopping paths for a successful parking. A corrective instruction for setting the correct stopping path in the form of an additional section is outputted only if the actual stopping path is situated outside the range of valid stopping paths.
According to example embodiments, the tolerance band or bands is/are dynamically adapted. The method is thus adaptive, so that for example the day-to-day condition of the driver can be taken into account.
According to example embodiments, the dynamic adaptation of the tolerance band for the stopping path takes into account a statistical variance of acquired stopping paths.
According to example embodiments, the dynamic adaptation of the tolerance band for the steering angle takes into account a statistical variance of acquired steering angle settings.
According to example embodiments, the size of the respective tolerance band can be selected corresponding to various driver categories.
According to example embodiments, on the basis of at least one criterion a successful parking process is defined ahead of time, the criterion permitting a multiplicity of final positions.
According to example embodiments, the criterion is selected from the following group: maximum number of passes, minimum pass length, maximum protrusion into the opposite lane of traffic, minimum or maximum distance from the curb, maximum distance to a reference line defined by adjacent parked vehicles, maximum angular deviation from the direction of the curb or to a reference line, minimum or maximum distance to the front and rear boundaries of the parking space.
Exemplary embodiments of the present invention are shown in the drawing, and are explained in more detail in the following description.